The invention deals with an apparatus for the transmission of signals and data for the control and monitoring of underwater pile drivers and cut-off equipment or similar work units from above the water surface to the underwater equipment lowered to below the water surface, or vice versa.
For more than 10 years underwater pile driving has been considered an established process in offshore foundation work. However, experience has shown that umbilicals, required for the operation of underwater pile drivers, are quite susceptible to damage. The deeper the underwater operation, the higher the costs incurred for repair or replacement of the long umbilical, and for downtime which alone can be as high as U.S. $ 17,000. per working hour.
This vulnerability is mainly due to the sensitivity of signal and control lines contained in the umbilical. Electrically controlled pile drivers are hardest hit, because they fail as soon as the electrical supply of the control equipment is impaired, or disruptions occur, because control processes have ceased. This is also true for electric control processes of automatically controlled equipment, except that its operation is not interrupted.
This vulnerability affects other underwater work units also, especially underwater cut-off equipment which will be utilized more often in the future under similar working conditions when redundant drilling platforms are removed.
Experience has shown that air pressure lines installed inside the umbilicals and electric power cables for operating a submerged underwater drive unit, rarely cause failures. However, some of the much thinner signal and control cables are often damaged and not useable, sometimes even before the operation begins.
This is because they are already damaged by the rough handling onboard and during the lowering of the equipment such handling causes the umbilical to hook, catch, or twist around objects onboard, around the underwater structure or the equipment itself. During the operation and subsequent raising of the equipment, they are further jeopardized.
Efforts to remedy the situation were concerned mainly with the umbilical design and the corresponding proving tests with the result that, no matter where and how the thin signal wires are arranged in the umbilical cross-section, they are always the weakest link between the rugged reinforcement, mostly designed for large tensile forces, and the other more rugged lines. Furthermore, an arrangement yielding a completely uniform load distribution can be achieved neither through design nor manufacturing technique. Their increased susceptibility to damage is therefore unavoidable.
Therefore many more signal wires are provided than required, a measure which is intended to prevent failure, but in fact only postpones it.
Damage to the umbilical and repair cost are accepted as unavoidable in practice. The cumulative cost of damage and downtime up to now are in the millions of dollars and spoil the economic viability of underwater work units, especially that of pile drivers. This is even more true when the failure occurs in an umbilical, which is used in the operation of an additional underwater drive unit mentioned before, and which may cost up to U.S. $ 1400 per meter, depending on the transmitted power.
Repairs made by patching damaged wires are only conditionally successful. They adversely affect the strength of the sheathing and weaken its ability to transmit tensile forces. Often patching is responsible for repeated failures with corresponding downtime. Furthermore, the diameter of the umbilical is considerably increased over a certain length in the vicinity of the patch, which can lead to difficulties when winding it on a drum, especially if there is a winding-on device on the drum.
In order to be able to complete the work at hand in such cases, one sometimes resorts to an additional cable installed parallel to the umbilical, containing the required signal wires. Apart from the fact that this cable and the accompanying cable drum and guide installation must be provided and installed additionally in the crane boom, additional personnel is also required. The cable causes problems because, due to its low specific gravity, it drifts in the water uncontrollably, unless it is provided with additional weight or connected to a steel rope. Such improvisations, made at the work site, are only possible when working in more shallow water depth.
All in all the current situation is highly unsatisfactory both technologically and economically.